This invention relates to improvements in photoelectric incremental measuring devices for measuring the position of a first object relative to a second object. In particular, this invention relates to improvements in measuring devices of the type which include a scale having a division grid and at least first and second reference marks spatially separated along the grid, each of the reference marks comprising a group of irregularly spaced reference lines, and wherein the device further includes a scanning plate having at least two scanning regions, each having a plurality of reading fields for reading the scale.
Measuring devices of this general type are known to the art. In West German unexamined patent specification OS No. 18 14 785, for example, there is described a photoelectric length measuring device in which the measuring scale defines both an incremental division grid as well as a reference mark. This document presents and describes in some detail the functioning of the disclosed measuring device. Furthermore, the functioning and construction of the reference mark is described in detail, particularly in connection with FIG. 2.
It is also known to the art, as for example in West German Pat. No. 876 162, that particularly useful reference marks are provided if each reference mark is made up of a number of lines arranged in a line group, and the separations between adjacent lines in each line group are unequal. When such reference marks are used, the scanning plate used to detect these marks includes a reading field having a line group with the same line distribution as that of the reference mark being detected. When exact and complete overlap is obtained between such a reference mark and the matching reading field, the intensity of light transmitted through the scanning plate and the measuring scale reaches a sufficiently high value to distinguish it unambiguously from noise. Thus, reference marks and corresponding reading fields of the type described above provide the advantage of particularly high signal-to-noise ratios.
It is well known that incremental measuring devices exhibit the potentially significant problem that once the operation of the device has been interrupted the absolute position of the measuring device cannot be directly recovered. West German unexamined patent specification OS No. 18 14 785 describes a structure and procedure for recalibrating an incremental measuring device in a particularly simple manner after operation has been interrupted.
Known photoelectric incremental measuring devices include at least one light source for illuminating the measuring scale and the scanning plate. When this light source fails to operate properly (as for example when the filament of an incandescent lamp breaks) the operation of the measuring device is interrupted, and this can lead to substantial delays while the lamp is replaced and the measuring device is recalibrated.